The present invention is directed to an irrigated tip catheter, and more particularly to a porous tip electrode design for an irrigated tip catheter.
Electrode catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity.
In use, the electrode catheter is inserted into a major vein or artery, e.g., femoral artery, and then guided into the chamber of the heart which is of concern. Within the heart, the ability to control the exact position and orientation of the catheter tip is critical and largely determines how useful the catheter is.
In certain applications, it is desirable to have the ability to inject and/or withdraw fluid through the catheter. This is accomplished by means of an irrigated tip catheter. One such application is a cardiac ablation procedure for creating lesions which interrupt errant electrical pathways in the heart.
A typical ablation procedure involves the insertion of a catheter having a tip electrode at its distal end into a heart chamber. A reference electrode is provided, generally taped to the skin of the patient. RF (radio frequency) current is applied to the tip electrode, and current flows through the media that surrounds it, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated sufficiently to cause cellular destruction in the cardiac tissue resulting in formation of a lesion within the cardiac tissue which is electrically non-conductive. During this process, heating of the electrode also occurs as a result of conduction from the heated tissue to the electrode itself. If the electrode temperature becomes sufficiently high, possibly above 60xc2x0 C., a thin transparent coating of dehydrated blood protein can form on the surface of the electrode. If the temperature continues to rise, this dehydrated layer can become progressively thicker resulting in blood coagulation on the electrode surface. Because dehydrated biological material has a higher electrical resistance than endocardial tissue, impedance to the flow of electrical energy into the tissue also increases. If the impedance increases sufficiently, an impedance rise occurs and the catheter must be removed from the body and the tip electrode cleaned.
In a typical application of RF current to the endocardium, circulating blood provides some cooling of the ablation electrode. However, there is typically a stagnant area between the electrode and tissue which is susceptible to the formation of dehydrated proteins and coagulum. As power and/or ablation time increases, the likelihood of an impedance rise also increases. As a result of this process, there has been a natural upper bound on the amount of energy which can be delivered to cardiac tissue and therefore the size of RF lesions. Historically, RF lesions have been hemispherical in shape with maximum lesion dimensions of approximately 6 mm in diameter and 3 to 5 mm in depth.
In clinical practice, it is desirable to reduce or eliminate impedance rises and, for certain cardiac arrhythmias, to create larger lesions. One method for accomplishing this is to monitor the temperature of the ablation electrode and to control the RF current delivered to the ablation electrode based on this temperature. If the temperature rises above a preselected value, the current is reduced until the temperature drops below this value. This method has reduced the number of impedance rises during cardiac ablations but has not significantly increased lesion dimensions. The results are not significantly different because this method still relies on the cooling effect of the blood which is dependent on location in the heart and orientation of the catheter to endocardial surface.
Another method is to irrigate the ablation electrode, e.g., with physiologic saline at room temperature, to actively cool the ablation electrode instead of relying on the more passive physiological cooling of the blood. Because the strength of the RF current is no longer limited by the interface temperature, current can be increased. This results in lesions which tend to be larger and more spherical, usually measuring about 10 to 12 mm.
The clinical effectiveness of irrigating the ablation electrode is dependent upon the distribution of flow within the electrode structure and the rate of irrigation flow through the tip. Effectiveness is achieved by reducing the overall electrode temperature and eliminating hot spots in the ablation electrode which can initiate coagulum formation. More channels and higher flows are more effective in reducing overall temperature and temperature variations, i.e., hot spots. The coolant flow rate must be balanced against the amount of fluid that can be injected into a patient and the increased clinical load required to monitor and possibly refill the injection devices during a procedure. In addition to irrigation flow during ablation, a maintenance flow, typically at a lower flow rate, is required throughout the procedure to prevent backflow of blood flow into the coolant passages. Thus reducing coolant flow by utilizing it as efficiently as possible is a desirable design objective.
One method for designing an ablation electrode which efficiently utilizes coolant flow is the use of a porous material structure. Such a design is described, for example, in U.S. Pat. Nos. 5,643,197 and 5,462,521 to Brucker et al. The use of a porous material in which tiny particles are sintered together to form a metallic structure provides a multiplicity of interconnected passages which allow for efficient cooling of an electrode structure. Additionally, the porous structure provides a uniform flow distribution of perfusate over the outside surface of the electrode which acts as a barrier layer between the blood and the electrode surface. This barrier layer prevents contact with the blood and the electrode further minimizing the possibility of coagulum formation on the catheter tip. This effect is important in regions where flow may be reduced by other structures such as temperature sensors and attachment methods.
Previously attempts have been made to design an irrigated tip catheter having a porous tip electrode made of a sintered material through which saline or other fluid can pass. However, Brucker et al. do not describe how various components, such as a thermocouple, lead wire or fluid tube, are mounted into the porous electrode. If these components are mounted in the traditional manner, i.e., by gluing or soldering them directly to the electrode, the glue or solder tends to seep into and through the porous material, thus reducing or even completely blocking the flow of cooling fluid through the porous material. Accordingly, a need exists for a porous tip electrode design in which various components can be mounted without blocking the flow of fluid through the porous material. In addition, the design of an irrigated tip electrode requires extensive testing at the bench and animal levels to verify that it provides the desired safety and clinical benefits, Accordingly, a need exists for a tip electrode design that reduces the complexity inherent during development of current cooled electrode designs.
In one embodiment, the present invention is directed to a porous tip electrode for a catheter. The electrode comprises a body and an insert. The body comprises a porous material through which fluid can pass and has a cavity therein. The insert comprises a non-porous material, is contained within the cavity, and has at least one passage extending therethrough. By providing separate structures for the mechanical and hydraulic functions, changes in catheter design, e.g., changes in diameter or number of lumens, the insert can be redesigned without altering the design of the porous body. This has the distinct advantage of requiring less testing to ensure the electrode provides its desired clinical benefit.
In a particlarly preferred embodiment, the body is in the form of a shell having a radius, a cylindrical sidewall, an open interior and a hemispherical cap at its distal end. Preferably the cylindrical sidewall of the shell has a uniform thickness. The porous shell is designed to achieve efficient cooling of the electrode structure and to maintain a uniform layer of perfusate around the outside surface of the electrode.
In another embodiment, the invention is directed to an irrigated tip catheter having a porous tip electrode. The catheter comprises a catheter body and a tip section. The catheter body has an outer wall, proximal and distal ends, and a lumen extending therethrough. The tip section comprises a segment of flexible tubing having proximal and distal ends and at least one lumen therethrough. The proximal end of the tip section is fixedly attached to the distal end of the catheter body. The porous tip electrode is fixedly attached to the distal end of the tubing of the tip section. The tip electrode has an outer surface and comprises a body and an insert. The body comprises a porous material through which fluid can pass and has a cavity therein. The insert, which comprises a non-porous material, is contained within the cavity of the body. The insert has at least one passage extending therethrough in fluid communication with a lumen in the tip section. An infusion tube having proximal and distal ends extends through the central lumen in the catheter body. The distal end of the infusion tube is in fluid communication with the proximal end of the passage in the tip electrode, whereby fluid can flow through the infusion tube, into the passage in the tip electrode and through the porous material of tip electrode to the outer surface of the tip electrode.